An Iranian navy boat fires a missile in a drill in the sea of Oman in 2011. (Ali Mohammadi / IIPA via the AP)

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Threats from Iran that it will attack the forces of the U.S. or any country that tries to take out its nuclear program are not mere boasts, say experts, but any such strike could be met with devastating counter-attacks.

Iran's high-flying ballistic missiles could overwhelm U.S. missile defenses in the Persian Gulf, where much of the world's oil passes. Its fast-attack boats could swarm a large warship and sink it. And its fleet of hard-to-find submarines carry torpedoes faster than any torpedo in the U.S. fleet.

But Iran cannot sustain an attack against the U.S., said Christopher Harmer, a former U.S. Navy commander who participated in war planning for U.S. forces in the Persian Gulf.

"The Iranian navy could hit us one time at sea or on shore. If they did that, we would eliminate all of their navy and probably most of their land-based missile capability," he said.

Iran has been expanding its defense and offensive capabilities with the help of China, Russia and other nations amid demands from the United Nations and Western nations that it prove its nuclear program is not for making a bomb in violation of its international agreements. Iran insists its program is for peaceful purposes.

Israel has said Iran will have bomb-making capability in as early as six months, and Prime Minister Benjamin Netanyahu has asked President Obama to set a "red line" for Iran's nuclear progress that could trigger a military attack.

U.S. Secretary of State Hillary Rodham Clinton said the U.S. would set no deadlines for Iran to comply with the U.N. inspections, and two weeks ago the White House declined to grant Netanyahu's request for a meeting on the matter.

The Obama administration said financial sanctions against Iran will persuade the Islamic nation to prove its nuclear program is peaceful by opening up to U.N. inspectors. But two years of sanctions have not done so. Iran President Mahmoud Ahmadinejad said this month the sanctions targeting the Iranian oil and banking sectors will never interrupt the country's nuclear program.

"We are going ahead, and God willing we will succeed," he said on Iranian state TV.

To protect against a strike on its nuclear facilities, Iran's Revolutionary Guard Corps is moving forward with an air-defense installation in the Abadeh area that would be the largest in that part of the country.

"If (the enemy) ever has the intent of attacking this soil, we will make the Persian Gulf their grave," said Abadeh's governor, Mohammad Javad Askari.

A sophisticated arsenal, however, has not been Iran's chief weapon. The U.S. State Department says Iran has significant influence over terrorist networks, which analysts say could easily target American civilians in the Middle East and Europe, and its civilian ships could launch poison-gas rockets onto the U.S. homeland.

State says Iranian terror proxies such as Hezbollah have killed hundreds of American citizens, including the 1983 attack that killed 241 US servicemembers at a barracks in Beirut, and the FBI said it uncovered an Iranian plot in October to kill Saudi Arabia's ambassador to the U.S. in Washington.

Iranian leaders and their allies have made "a lot of statements" that they would attack U.S. targets if Israel hit Iran's nuclear program, said Richard Wachtel of the Middle East Media Research Institute, a Washington-based translation service.

The latest was Sunday, when Gen. Amir Ali Hajizadeh of the Revolutionary Guard Corps told Iran's Al-Alam TV that U.S. targets would be fair game in an Israeli attack because it wouldn't happen without U.S. approval. U.S. facilities in Bahrain, Qatar and Afghanistan would be targeted, he said.

U.S.military planners worry most about a few basic weapons that Iran has obtained from other countries, like Russia, China and North Korea, and then improved.

• Missiles — Land-based cruise missiles could target U.S. warships and the U.S. Navy's 5th Fleet headquarters across the Persian Gulf in Bahrain. U.S. ships and bases have anti-missile defenses, but they could be overwhelmed if Iran shoots enough missiles at the same target at once, Harmer said.

• Super-fast Hoot torpedoes — Based on a Russian design, these rocket-propelled torpedoes can be shot from a submarine and travel 200 mph, faster than any torpedo in the U.S. arsenal.

While a ship has a reasonable chance of evading a traditional torpedo, "there's no chance of a ship evading a rocket propelled torpedo," Harmer said.

• Mini-submarines — At least 20 mini-submarines that are hard to track and easy to hide if they sit still on the seafloor can target passing U.S. ships and possibly sink them. The U.S. Navy uses satellites and other means to monitor and destroy them, if necessary, but the subs are small and can remain hidden for weeks at a time.

• Fast-attack patrol boats — Iran has hundreds of fast boats armed with missiles and loaded with explosives to swarm the Strait of Hormuz while an aircraft carrier group is passing through.

To counter the threats, the Pentagon has bolstered the U.S. defenses in the Persian Gulf as a preemptive measure to counter an attempt by Iran to close the Strait of Hormuz. U.S.attack helicopters and aircraft routinely patrol the waters around aircraft carriers. U.S. Navy ships are armed with a range of guns that would penetrate the thin-hulled fast boats and sink them, Harmer said.

Harmer agrees that a swarm of small boats may overwhelm and sink a U.S. warship, but, "They could do that one time, and then the U.S. Navy would essentially destroy the Iranian navy," in as little as four days, he said.

John Pike, director of the defense think tank Globalsecurity.org., said Iran also has chemical weapons that it could launch from civilian ships at U.S.' East Coast. While U.S. policy since 2010 is not to respond to chemical weapons attacks with nuclear weapons, Iran and North Korea are exempt from that policy, according to the Union of Concerned Scientists.

Michael O'Hanlon, a scholar at the Brookings Institution who serves on the CIA's external advisory board, said Iran's terror capabilities are widespread and could be employed quickly if desired.

"They're capable of doing that in a couple dozen countries across the Middle East," and could kill dozens to hundreds of vulnerable American travelers, embassy workers and business people, O'Hanlon said.

The U.S. could respond with airstrikes on Iranian military installations, he said. But, "There's no way to know where this would end or who would get hurt worse," O'Hanlon said.